The energy delivered by a defibrillator is typically expressed in Joules. Energy has become a surrogate for current in modern defibrillator language, but electrical current is what actually defibrillates the heart. A Joule is the unit of work associated with one amp of current passed through one ohm of resistance for one second. Electrical current may be expressed as the ratio of voltage/impedance. Both the patient and the electrical circuit of the defibrillator contain impedance, which is expressed in ohms
A multivector shock is one which utilizes three or more therapy electrodes to deliver electrical current along at least two separate current paths. Studies have shown that multivector shocks can substantially reduce the energy required for defibrillation. For example, Pagan-Carlo et al. (JACC 1998) demonstrated that a multivector shock can defibrillate pigs with about 40% less energy than a standard biphasic truncated exponential (BTE) waveform. FIG. 1 is an illustration of the electrode positions used in this study and the timing of the associated waveforms. The authors used three electrodes to generate a series of 7 different shock vectors. According to this method, current is initially delivered from electrode 1 to electrode 2. This continues for 1 ms, then current is delivered from electrode 1 to electrodes 2 and 3 together. This continues for another millisecond. Then current is delivered from electrodes 1 and 2 to electrode 3. The shock sequence continues to switch between electrode combinations (i.e. “vectors”) until all seven vectors have been energized.
A circuit for delivering a three electrode multivector shock is shown in FIG. 2. Note that this implementation delivers a multivector shock from a single capacitor. In this respect, it is different from the configuration disclosed by published patent application US 2005/0107834, entitled “Multi-path Transthoracic Defibrillation and Cardioversion”. As shown, the circuit of FIG. 2 contains three half-bridges, one for each electrode. If more electrodes were utilized, more half-bridges would be required. Each of the switches could be a semiconductor switch such as an IGBT (insulated-gate bipolar transistor), SCR (silicon-controlled rectifier), MOSFET (metal-oxide-semiconductor field-effect transistor), BiMOSFET (bipolar MOSFET), or similar device. The switches in the FIG. 2 circuit are operated sequentially to deliver current to different combinations of electrodes as described above.
While Pagan-Carlo achieved good results with their shock sequence of 7 vectors, it is not clear whether that particular sequence is optimal or necessary. A smaller number of shock vectors may be equally efficacious as well as being easier to implement. For example, two shock vectors (electrode 1→electrode 2, followed by electrode 1→electrode 3) may be appropriate. In other words, a shock sequence with two or more vectors may be superior to a standard single-vector shock.
Historically, in defibrillator applications, multivector waveforms have been unattractive because of the difficulty of properly applying three or more electrodes to a patient. When a rescuer is treating a cardiac arrest patient, time is of the essence. The extra time required to apply extra defibrillation pads could be critical for survival. Applying a back pad is particularly difficult to do to a patient that is unconscious. Also, training rescuers to apply pads in novel, new locations would be difficult and confusing.
Many of the obstacles that complicate the application of a multivector shock can be overcome by a carefully designed wearable defibrillator. A wearable defibrillator may include a garment (such as a vest), a belt, or other arrangement that provides access to multiple points on the patient's thorax. This garment may allow multiple therapy electrodes to be easily placed in advantageous locations without extra hassle or confusion. Because the patient is conscious it is not difficult to access the patient's back.
A multivector defibrillator may be particularly advantageous for a wearable defibrillator because it may allow the use of smaller therapy electrodes. Pagan-Carlo proposed an “overlapping encircling” shock scheme that uses the available electrodes in different combinations to create a rotating shock vector that sweeps around the heart. This scheme uses electrodes in pairs and threesomes to create seven different shock vectors.
For a conventional two-electrode (single vector) defibrillation shock the size of the pads plays a role in ensuring the proper current distribution through the heart. In order for the shock to be successful, enough of the heart must receive enough current to depolarize the heart cells. Pads that are too small may not deliver enough current to some parts of the heart. In contrast to a conventional defibrillator, a multivector defibrillator may be able to use smaller pads because current flow through all parts of the heart is guaranteed by the multivector waveform not by pad size alone.
Therapy electrodes for multivector defibrillation might be applied to the patient in different arrangements. Pagan-Carlo suggested that the three pads be equally distributed around the perimeter of the thorax. This could be implemented using two anterior pads, one approximately beneath each breast, and one posterior pad in the middle of the back. Alternatively, there could be one anterior pad over the sternum and two back pads. Although Pagan-Carlo's arrangement worked well for pigs it is unclear whether it would be ideal for humans. The cross-section of a pig's thorax is much more cylindrical than a human's thorax and electrodes placed laterally on a human could deliver current relatively far from the heart.